1. Field of the Invention
The invention relates to a method for stepless capacity control of a reciprocating piston compressor whereby an unloader arranged on at least one automatic suction valve of the compressor keeps open at least one sealing element of the suction valve throughout a thereby controllable portion of the working cycle of the compressor through a switchable control valve having an unloading piston (draw piston) biased by gas pressure. The invention relates further to a corresponding reciprocating piston compressor with stepless capacity control having an unloader attached on at least one automatic suction valve of the compressor whereby the unloader keeps open at least one sealing element of the suction valve throughout a thereby controllable portion of the working cycle of the compressor by means of an unloading piston biased by gas pressure via a switchable control valve.
2. The Prior Art
Compressors known to have also reverse flow controls with stepless capacity control of the described type are known in the art. See in this regard U.S. Pat. No. 2,296,304 A, U.S. Pat. No. 2,626,100 A or U.S. Pat. No. 5,378,117 A, for example. In all known methods and devices of the aforementioned type, there is already set or adjusted the engagement force of the unloader influencing the sealing element of the suction valve via an unloading cylinder biased by gas pressure or the pressure biasing the unloading piston therein. Up to now, this pressure has always been essentially constant or has been adjusted by a pressure regulator or also by pulsating switching control valves.
This type of capacity control takes advantage of the fact that the engaging flow force, which exists during the compression stroke at the sealing element of the suction valve that is kept open by the unloader—and which is thus termed as reverse flow force—increases at first with the progressing crank angle during the compression stroke, passes a maximum that corresponds to the piston velocity, and advances at the end of the compression stroke toward zero when reaching the upper dead center of the piston. Through an adjustment of the unloading force biasing the unloading piston by means of a prorated amount of gas pressure in the unloading cylinder, there can be determined the crank angle at which the unloading force is overcome by the reverse flow force (together with the possible spring action of the sealing element) whereby the arrangement consisting of the open sealing element and the unloader is accelerated in movement in the closing direction of the suction valve. The crank angle for closing of the suction valve can be adjusted in this manner in a continuous (infinite variable) manner between the lower dead center and the crank angle corresponding to the maximum of the reverse flow force (and the corresponding delivery amount of the compressor can be adjusted thereby.)
It is a direct disadvantage in the described method or the corresponding disclosed devices in that the closing crank angles, existing after the arrival of the maximum reverse flow force, can naturally not be realized, which results in a limited range of control that lies approximately between 40 to 100 percent of the maximal possible delivery amount. Especially in the production of PET (polyethylene) bottles, there are nevertheless a great number of air compressors employed, for example, which experience a highly fluctuating air requirement of 10 to 100 percent and which must maintain a very constant end pressure at the same time.
It is an additional disadvantage that the required gas pressure necessary for the adjustment of a specific delivery amount and directly influencing the gas pressure in the unloading cylinder depends on many parameters, such as gas density, operational pressure, speed of the compressor and the like, which results in additional complicated and failure-susceptible control methods or control mechanisms.
Another known method for capacity control in compressors is the intermittent operation of the compressor (on/off control) whereby the suction valves are alternately kept open by means of unloader actuation or whereby they are permitted to open and close automatically. This control by unloader actuation can be basically used for adjustment of an average delivery amount between 10 percent and 100 percent but it causes various additional disadvantages: The compressor runs alternately at full power or idle. While running idle, the unfavorable degree of effectiveness and the high phase shift of the three-phase A.C. motor employed to drive compressors lead to high energy consumption or large amounts of reactive current (idle current). The sealing elements of the rod packing are not being scavenged by gas leakage during idle operation and are thereby not cooled, or the heat developing at the open suction valves by the lack of ventilation is not dissipated via the delivery medium. The thereby developing increase in heat and the deformation of the sealing element as a result of temperature changes promotes the wear of ring components and packing components.
Aside from the problems with rings and packing, this type of control is also responsible for damages to the valves. The reciprocal movement of the unloader over conventional diaphragm cylinders or other cylinders is possible only within several compression cycles based on the large volumes, the clearance volume, the small cross section of the inlet lines, the great length of the lines, the small cross section of the switch and the long switch-over times of the control valves. The sealing element of the suction valve, normally a valve plate, impacts the unloader prongs several times during the reciprocal movement. This can accelerate or initiate the breaking of valve plates.
Constant pressures in the pressure reservoir of compressors having conventional on/off controls are dependent on the reservoir volumes and may be realized only by frequent switching between idle operation and operation under full power (several times per minute.) Components of the piston cylinder and of the diaphragm cylinder are generally not suited for frequent switching and are subject to increased wear.
Methods and devices have been disclosed to avoid the described disadvantages whereby a unloading force is provided by hydraulic means acting upon the unloader against the reverse flow force of the gas to be compressed whereby said unloading force is suddenly reduced at a specific crank angle and whereby secure and rapid closing of the suction valve is initiated. Such devices, as disclosed in AT 403 835 B, for example, use systems for this purpose which are highly suitable based on the low compressibility of the employed actuation fluids, but which have the disadvantage that they are designed relatively complicated and that they need additionally an hydraulic assist energy that must be provided through additional aggregates.
It is the object of the present invention to improve the reverse flow control actuated by means of gas pressure of the aforementioned type in such a manner that the cited disadvantages do not occur, particularly to avoid in a simple way the above-mentioned limitations in the range of control as well as the negative influences of fluctuations in the necessary unloading gas pressure.